A) The Drosophila mutant which possesses an altered DNA polymerase alpha has been further characterized. Similar types of DNA polymerase mutants have been isolated from the yeast Saccharomyces cerevisiae and it has been proven that DNA polymerase I is a true DNA replicase in yeast. B) An in vitro DNA replication system of yeast 2 Mum plasmid has been developed from a crude extract of S. cerevisiae. The in vitro origin and direction of plasmid 2 Mum DNA replication have been determined and are the same as in vivo. C) In order to understand the importance of DNA tertiary structure for DNA replication, DNA topoisomerases, which might play an important role in DNA replication, have been purified from human and insect cell lines and studied extensively.